The invention relates to means for energy transmission and distribution and more particularly, to a dual series-feed network usable for antennas.
Typically, a dual series, center-fed, waveguide ladder network for feeding a monopulse array antenna consists of a symmetrical network having a primary line, a secondary line, N crossguide lines, N phase shifters, and 2N crossguide directional couplers; where N is the number of crossguide lines coupling the primary and secondary lines to the antenna elements. Typically, the primary and secondary lines are parallel to each other and the crossguide lines are perpendicular to and interconnect both the primary and secondary lines.
In the typical application, coupling devices feed the primary line with the sum excitation signal and feed both the primary and secondary lines with the difference excitation signal. Excitation of each antenna element in the transmit mode of operation is obtained by using directional couplers to sequentially tap off power from the primary and secondary lines and using the crossguide lines to conduct that power to the antenna elements. In the transmit mode, the sum and difference excitation signals are independently generated and applied to the primary and secondary lines as described above so that with the introduction of the proper phase shifts, the desired power distribution for radiation from the antenna elements is created. In the receive mode of operation, this process works in reverse. The received power distribution from the antenna elements is subjected to respective phase shifts, if any, and is conducted to the primary and secondary lines by directional couplers. In the primary and secondary lines, the received, phase shifted energy is separated vectorially into the sum and difference excitation signals by the center feed coupling devices.
Feeding the independent sum and difference excitation signals to the primary and secondary lines as described above creates in each a pair of "phase reversal points," one of the pair being located on one side of the center feed device and the other of the pair being located on the other side of the center feed device. At these phase reversal points, the amplitudes of the sum and difference signals are equal in magnitude.
In order to create a difference power distribution at the antenna elements, a phase inversion is introduced beyond the phase reversal points to result in the desired vectorial addition of the signals. A prior technique for introducing this 180.degree. phase differential is the inclusion of a dielectrically loaded, waveguide-section type phase shifter in each of the crossguide lines disposed after this phase reversal point. In this technique, the crossguide lines disposed between the center feed device and the phase reversal point impose 180.degree. phase shift while those crossguide lines outside of the phase inversion point impose 0.degree. phase shift.
In this prior embodiment, crossguide lines were coupled to both sides of the primary and secondary lines thus forming a structure resembling a "double ladder." That is, a first crossguide line would link the first broad wall of the primary line with the first broad wall of the secondary line and a second crossguide line would link the second broad wall of the primary line with the second broad wall of the secondary line. Therefore, the first and second crossguide lines were on opposite sides from each other of both the primary and secondary lines. Their locations on those sides, however, are restricted. Locating the phase shifters in the crossguide lines resulted in a potential for interference among the phase shifter fields. Consequently, crossguide lines could not be located directly opposite one another, and in most cases could not even overlap. They would be staggered in relation to one another to avoid possible phase shifter interference. This requirement of staggering resulted in the crossguide lines on the same side of the main lines being spaced relatively far apart because enough room had to be reserved between them to accommodate the respective crossguide lines on the opposite side. As a result, when this arrangement was utilized in prior radar applications, such as in a monopulse radar system, the resolution of the system was degraded.
A further problem exists with the above technique where a phase shifter is disposed in each crossguide line. Due to signal perturbations and inaccuracies in the phase shifters, they must be individually tuned relative to the neighboring phase shifters to maintain the 180.degree. phase difference on either side of the phase reversal point. As is well known to those skilled in the art, individual tuning is time consuming and relatively expensive. Further disadvantages of this technique include the relatively heavy weight of the feed resulting from the use of so many phase shifters, the expense of so many phase shifters, and the manufacturing difficulties involved in installing a phase shifter in each crossguide line. Another disadvantage is the relatively large size because of the physical dimensions of the matching and transition structures that are required in each crossguide feed line for well matched phase shifters. In this prior technique, not only are the crossguide feed lines located relatively far apart due to the staggering requirement, but the primary and secondary feed lines are also located relatively far apart to accommodate the size of the phase shifter and its transitions and matching devices used in each crossguide feed line. This overall large size made the prior technique network unsuitable for many airborne applications.
It is an object of the invention to provide a dual series waveguide feed network which overcomes most, if not all, of the above disadvantages of the prior techniques.
It is also an object of the invention to provide a dual series feed network which uses fewer phase shifters than prior techniques but which has performance characteristics equal to or better than prior techniques.
It is also an object of the invention to provide a dual series feed network which does not require the location of phase shifters in the crossguide lines, but which uses only a pair of phase shifters located in the secondary line to achieve the desired energy distribution at the antenna elements.
It is also an object of the invention to provide a dual series feed network which locates the crossguide lines closer together and the primary and secondary lines closer together than in prior techniques thereby offering more compactness and higher resolution in certain applications.
It is also an object of the invention to provide a dual series feed network which uses double crossguide couplers to couple the crossguide lines opposite one another to achieve greater compactness.